1. Technical Field
This disclosure relates to antilock brake systems and, more specifically, to electrohydraulic antilock brake systems for articulated vehicles.
2. Description of the Related Art
Braking a vehicle in a controlled manner under adverse conditions such as rain, snow, or ice generally requires precise application of the brakes by the vehicle driver. Under these conditions, or in panic stop situations, a driver will often apply excessive brake pressure, thereby causing the wheels to lockup and slip or skid on the road surface. Wheel lockup conditions can lead to loss of directional stability and, possibly, uncontrolled vehicle spinout.
In a continuing effort to improve the operational safety of vehicles, antilock braking systems have been developed. While such systems are adapted to control the braking of each braked wheel of a vehicle, some systems have been developed for controlling the braking of only a portion of the braked wheels. Generally, antilock brake systems are electrohydraulic and include a controller and sensors for monitoring the speed of the controlled wheels to determine the deceleration of the controlled wheels. Antilock brake systems also include one or more hydraulic circuits for applying pressure to the brakes of the controlled wheels. When the brakes of the vehicle are applied and the wheel deceleration of the monitored wheels exceeds one or more of predetermined thresholds, indicating that there is wheel slippage and the wheels are approaching a lockup condition, the controller functions to control the application of hydraulic pressure through a series of valves associated with the brakes to prevent a lockup of the controlled wheels. Typically, the controller will deactivate and activate the valves to cyclically release and reapply pressure to the brakes to limit wheel slippage to a safe level while continuing to produce adequate brake torque to decelerate the vehicle as desired by the driver.
In the interest of cost reduction, ABS systems have been deployed where the number of control channels is less than the number of sensed wheels. For example, some manufacturers have explored the concept of “dual-channel” or “dual-circuit” antilock brake systems which control all rear wheel brakes with one channel of control and both front wheels with another channel of control. Dual-circuit hydraulic antilock brake systems include a pair of hydraulic brake circuits which are hydraulically independent of each other. One brake circuit is adapted to establish a braking pressure to be applied to the front wheel brake cylinders, while the other brake circuit is adapted to establish another independent braking pressure to be applied to the rear wheel brake cylinders. An example of such a dual-circuit hydraulic antilock brake system is disclosed in U.S. Pat. No. 4,824,183.
However, prior art ABS systems that minimize the number of control circuits may introduce problems. For example, the surfaces on which the left and right wheels are rotating may be different, e.g., one wheel may be rotating on ice patch and the other wheel may be rotating on dry asphalt. Some ABS systems may allow one wheel to lock while the other wheel rotates in order to minimize the stopping distance. However, directional control is compromised, especially if one front wheel is locked while the other front wheel is rotating. The tire that is locked is also subject to unnecessary wear and, for off-highway machines, the locked tire may be damaged or destroyed if it is dragged over a sharp object. Other ABS systems respond by not permitting the one wheel to lock, but the braking torque applied to both wheels, including the rotating wheel, is reduced. Obviously, this strategy compromises stopping distance. Yet other ABS systems compromise between these two concepts and allow one of the wheels to lock for short periods of time before reducing the braking torque applied to both wheels for short period of time before the braking torque is increased again, which may cause one of the wheels to lock again. The process is then repeated. As a result, directional control and stopping distance may be compromised.
Thus, new ABS systems are needed that limit the number of control circuits thereby reducing costs but also avoid locking of the braked wheels for improved control and also limit brake torque reductions to improve stopping distances.